1. Field of the Invention
This invention relates to coated particles and a coating method, and particularly to coated particles applied to a phosphor screen of a display as phosphor particles, and a method for forming such coated particles.
2. Description of Related Art
Recently, a display screen of a display apparatus and a cathode ray tube (hereinafter referred to as a CRT) have been promoted because of their flatness and large-size, and improvements in their resolution and image quality are expected.
For example, as is well-known, a CRT has a glass panel and a glass funnel that are integrally formed to be a tube, and an electron gun is provided inside the tube. Further, inside the panel of the tube, a color selection mechanism is provided, and an inner surface of the panel is formed to be a phosphor screen.
Black-colored, light absorption material film patterns made of carbon are formed on the inner surface of the panel as the phosphor screen. Among the light absorption material film patterns, a red pigment layer for transmitting red light, a green pigment layer for transmitting green light, and a blue pigment layer for transmitting blue light are formed in pattern respectively as color filters of red, green, and blue. Further, a red phosphor particle layer for emitting red light, a green phosphor particle layer for emitting a green light, and a blue phosphor particle layer for emitting a blue light are formed in pattern respectively on each of corresponding color pigment layers.
In such a CRT, an electron beam (cathode ray) emitted from an electron gun passes a color selection mechanism while being color-selected and excites the phosphor particles of each color at the red phosphor particle layer, the green phosphor particle layer, and the blue phosphor particle layer. Thus, the phosphor particles excited by the electron beam emit fluorescent light, and thereby a corresponding color image is displayed on an outer surface of the panel.
As a characteristic of a display apparatus, such as a CRT or the like, image quality is very important, and the contrast of the image determines the image quality. Colors of respective red, green, and blue phosphor particles provided on the phosphor screen are approximately white, and the reflection rate for external light at the phosphor screen of the panel is relatively high, so that this causes one of the reasons for decreasing the contrast of the display image. Accordingly, in order to increase the contrast, reflection of the external light is suppressed by providing color filters of red, green, and blue, as above mentioned.
However, in the cathode ray tube as configured above, it is necessary to form the pigment particle layers of each color before forming the phosphor particle layers on the inner surface of the panel in its fabrication process. This invites an increase in fabrication processes and a new investment for infrastructure in the case of in-line fabricating, so that it is not favorable.
Further as another method, it is known that pigment particles are directly coated on each surface of the phosphor particles in advance, and a layer of coated phosphor particles is formed on the phosphor screen. As one of the coating methods of applying the pigment particles to the surfaces of the phosphor particles, it is known that the phosphor particles and the pigment particles are dispersed in a dispersion medium, and then the pigment particles are fixed on each surface of the phosphor particles by adjusting the type of additive agent, their amount, and their pH. In this case, a mean particle diameter of the phosphor particles is around several μm, and a mean particle diameter of the pigment particles is around 100 to several 100 nm. Because the pigment particles are coated on each surface of the phosphor particles, the reflection of the external light is suppressed and improves the contrast of the display image.
In the case of the above mentioned coating method where the coating of the pigment particles to each surface of the phosphor particles is carried out in a liquid phase, it is necessary for the phosphor particles and the pigment particles to exist within the same liquid phase. Accordingly, an aggregation of particles tends to take place due to differences in zeta-potentials based on material differences, and it is very difficult to carry out a uniform coating. Further, there is the problem that the pigment particles exist non-uniformly on surfaces of the phosphor particles. If the pigment particles are non-uniformly coated on the surfaces of the phosphor particles, the emitting efficiency of the phosphor particles is decreased, and simultaneously, the reflection property of the phosphor particles is deteriorated. In the liquid phase where the phosphor particles and the pigment particles are mixed, the less a particle diameter becomes, the more the aggregation of particles occurs, so that it was practically impossible to use pigment particles having a particle diameter of less than 100 nm.
As described above, the method for coating more microscopic particles, such as pigment particles, on surfaces of microscopic particles, such as phosphor particles, has got a lot of attention as a surface property refining technology of particles, but the main stream is to use a liquid phase, so that it is difficult to carry out an uniform coating. Refining of the surface property of particles is very important technology. Technology for giving a new functionality to particles has wide application in various fields, such as the biological electronic materials fields, so that a method for uniformly coating the surfaces of microscopic particles with more microscopic particles is expected.
However, in the coated particles being coated with the pigment particles on each surface thereof by the above-mentioned conventional method, the pigment particles are likely to be peeled off the surfaces of the phosphor particles. Thus, there is a problem where the pigment particles peeled off from the surfaces of the phosphor particles are clogged in pipes of a fabrication apparatus during a phosphor screen fabrication process.